This invention relates to pneumatic conveyors for particulate material, and especially to pneumatic conveyors suitable for transferring particulate synthetic plastic materials from a storage hopper to an injection molding machine or other processing apparatus.
Pneumatic conveying systems have found general application in conveying powders and other particulate materials over substantial distances. The most commonly employed systems of this kind are high pressure conveyors in which the particulate material is supplied to a conveyor conduit through a rotary valve or other similar isolation device. In the conduit, high pressure air carries the particulate material to its destination. A system of this kind requires high volumes of air at relatively high pressures, especially if the material being conveyed is cohesive enough to exhibit a substantial tendency toward agglomeration or "bridging." For a relatively cohesive material, the pressure requirements may be quite excessive, particularly when it is considered that the pressure required varies as the square of the continuous length of the material in the pipe.
In another pneumatic conveyor system that has found particular application with relatively cohesive materials, the material is divided into discrete plugs separated by spaces filled with air. In one system of this kind, described in Flain et al U.S. Pat. No. 3,604,758, the principal volume of high pressure air for operation of the conveyor is introduced into the top of a sealed storage hopper. A limited additional volume of air is continuously supplied to the hopper at a point near its outlet connection to the conveyor conduit, which is maintained continuously open. The division of the particulate material into discrete plugs is effected by an air knife incorporated in the conveyor conduit at a point near to but spaced from the hopper outlet. A system of this kind can be constructed to work quite satisfactorily with a wide variety of particulate materials, including those having relatively high cohesive or "bridging" characteristics, but it does require rather precise control of the quantities of air introduced at three different points in the system, particularly with respect to control of the air knife.
Another system for dividing the particulate material into discrete plugs, which uses two series-connected valves at the hopper outlet, is described in Powning U.S. Pat. No. 3,874,560. In this system, an electrically controlled mechanical valve releases particulate material from a gravity flow hopper into a valve chamber on a cyclic basis. The outlet of that chamber is another valve, actuated by a pressure differential. When the inlet to the valve chamber is closed, air is introduced into the valve chamber under pressure to open the outlet valve and drive the particulate material into a conveyor conduit. The second valve imposes distinct limitations upon operation of the system and may result in undesirable pressure losses.